Bioresorable compositions such as polylactides (PLA) are useful for bone fixation and bone repair and have the advantage of not requiring surgical removal after the bone heals. However, the use of polylactides for bone fixation and bone repair can lead to a variety of undesirable side effects, such as inflammation or allergic reactions.
Combining apatite, such as hydroxyapatite (HA), with PLA yields a composition similar to the composition found in bone and teeth in vivo. Polylactide/hydroxyapatite (PLA/HA) composites facilitate the osteoconductive properties of an implant plus aides in lessening the side-effect of the PLA composite by neutralizing its acidic bio-degraded by-products. PLA/HA composites have the potential of improving clinical bone healing, but current PLA/HA composites have a significant disadvantage due to their mechanical weakness. This weakness eliminates the use of PLA/HA composites in load-bearing areas. It is generally suspected that PLA/HA composite weakness is caused by the weak interphase between PLA (hydrophobic) and HA (hydrophilic) structures.
Current PLA/HA composites are prepared by several different methods such as direct blending using nonmodified HA, solution co-precipitation, emulsion, and mechanical mixing. Because of the relatively high hydrophobicity of PLA and hydrophilicity of HA, obvious problems of these methods include weak interfacial adhesion between HA and the PLA matrix and agglomeration of the HA particles in the matrix. Lack of adhesion between the two phases will result in an early failure at the interface between PLA and HA, usually leading to weak mechanical properties. As an example, the tensile strength of PLA/HA composites decreased significantly from 54 MPa for pure PLA to 41 MPa even with a HA content of only 18%.
Increasing interfacial bond strength between PLA and HA is an important factor on a matrix interface to achieve increased mechanical strength. In the last decade, coupling agents such as silanes, isocyanates, and organotitanates have been used to improve the interfacial adhesion between certain ceramic fillers and different polymeric matrices. Although the effects on alumina and silica systems (SiO2, bioglass, clay, etc.) were encouraging, the feasibility of using these agents to gain improved interfacial adhesion to HA was not confirmed.